This invention relates to shielding tapes or use in telecommuication and/or television shielded cables.
Telecommunications cables as referred to herein are of the type including one or more insulated conductors which are designed to transmit low power, high frequency signals.
Cables for CATV uses generally comprise a single insulated conductor enclosed in an insulating plastic material. The conductor is shielded by a metallic braid, and enclosed in a protective plastic jacket. Such cables often include shielding consisting generally of a plastic film/foil laminate or multiple layers thereof. In some cases the laminate is bonded using a bonding agent which may be activated by heat or other means to the insulating material of the central conductor and/or to the outer jacket.
In the telecommunications cables for the transmission of television and similar high frequency signals, it is essential to shield such signals from losses occurring due to radiation, which reduce signal strength and also may cause interference to nearby electronic apparatus. Furthermore, it is necessary to protect the signal from external electromechanical interference, and radio frequency interference (EMI and RFI). Consequently, such conductors must be shielded. This has been done, where requirements are not stringent, by employing a braided return conductor or shield of copper or aluminum wire, but because of the openings in the braid, such shielding is not completely effective, even if multiple braids are employed. Furthermore, the cable may become stiff, hard to handle, and excessively costly and heavy.
This has led to the wrapping of the cable core with a metallic foil layer, either helically or longitudinally. While this provides better shielding, especially in conjunction with a surrounding metallic braid, foil by itself is nearly impossible to apply satisfactorily unless excessively thick, which in turn results in undue stiffness, unwanted weight and added cost in the cable. In turn, foil of a thinner gauge requires a support layer of non-conducting film laminated thereon to provide structural strength in application and service.
To further improve the shielding capability, a triple laminate of foil/plastic film/foil has been employed which reduces losses and interference, and also acts as a fail-safe in the situation of cracking of one of the foil layers when the cable is flexed in installation and operation. However, unless the foil layers are connected electrically, signal loss can occur through the "slot" created by the plastic film layer.
Attempts to alleviate the above problems have been recorded by Kincaid (U.S. Pat. No. 4,406,914 Sept. 27, 1983) in which the foil layer is laminated to the plastic film layer, following which the laminate is slit and then folded at one edge resulting in a quadruple laminate of foil/plastic/plastic/foil. Wilkenloh (U.S. Pat. No. 4,117,260 Sept. 26, 1978) teaches a foil/film/foil laminate which when wrapped around the cable core provides 720.degree. electrically continuous coverage but still does not provide true circumferential continuity and fully eliminate the "slot effect".